1. Field of the Invention
This invention relates to a magnetic encoder, and more particularly to providing an adjustable gap between a magnetic drum and a magnetic sensor.
2. Description of the Prior Art
FIG. 10 shows a conventional magnetic encoder which is disclosed in Japanese Utility Model Laid-Open No. 62-3012.
The magnetic drum 1 serving as a magnetic scale is made of a magnetic material and has a magnetization pattern formed on an outer circumferential periphery thereof. The magnetization pattern is formed by a plurality of magnetic pole pairs arranged at equal circumferential intervals such that adjacent successive pairs of magnetic poles have the polarities N (north), S (south), S, N, N, S, S, N, . . . . The rotary shaft 3 extends outwardly from the rotating machine 2 and is securely mounted at the center of the magnetic drum 1. A step 2a serving as a datum clamp face is formed on the surface of the rotating machine 2, and the base member 4 is screwed to the step 2a. The base member 4 has a disk-like configuration and has an upper reference face 4a extending perpendicularly to the axis of the rotary shaft 3. The retaining member 5 is screwed to the reference face 4a of the base member 4. The magnetic sensor 6 which may consist of a pair of magneto-resistance effect elements is attached to the retaining member 5 so that it faces and is spaced from the circumferential face of the magnetic drum 1. Components 11 of an electric circuit for processing an electric signal from the magnetic sensor 6 are carried on the printed circuit board 7. The printed circuit board 7 is secured to the base member 4 by means of a plurality of screws 12 extending through the support tubes 9 and is located above the magnetic drum 1. The lead line 8 is connected at an end thereof to the magnetic sensor 6 and at the other end thereof to the printed circuit board 7. The cover 10 is mounted on top of the rotating machine 2 so as to prevent dust from entering the magnetic drum 1, magnetic sensor 6 and so on.
The two magneto-resistance effect elements of the magnetic sensor 6 are arranged in a circumferentially spaced relationship and separated by a distance equal to np+1/4.multidot.p where p is a pitch of the magnetic poles of the magnetization pattern on the magnetic drum 1 and n is an integer. These elements also are arranged in an opposing relationship to the magnetization pattern such that magnetic paths thereof may be directed perpendicularly to the rotary shaft 3. When the magnetic drum 1 is rotated in a predetermined direction by the rotating machine 2, the magneto-resistance effect elements of the magnetic sensor 6 develop two continuous signals having a difference in phase equal to 90 degrees. The two signals are conveyed via the lead line 8 to the printed circuit board 7 where they are processed for amplification, detection and matching to produce incremental pulses from which the displacement and the direction of rotation of the magnetic drum are determined.
In a magnetic rotary encoder as described above, the retaining member 5 is screwed to the base member 4 and the gap between the magnetic sensor 6 and the magnetic drum 1 must be adjusted properly in order that the magnetic sensor 6 may reproduce the magnetization pattern of the magnetic drum 1 with high sensitivity. Unfortunately, the setting of the gap may be upset as the screw is tightened or when the screw is loosened by vibrations from the outside during use. Therefore, a bonding agent is sometimes used to secure the retaining member 5 to the base member 4. However, where a bonding agent is used as a securing means for the retaining member 5, the setting of the gap may be upset by expansion or contraction with temperature of the materials used. It is to be noted that the problem is not recognized by Japanese Utility Model Laid-Open No. 62-3012 mentioned hereinabove.
Further, in the conventional magnetic rotary encoder described hereinabove, to mount the printed circuit board 7 above the magnetic drum 1, the plurality of support tubes 9 are interposed between the printed circuit board 7 and the base member 4 and the comparatively long screws 12 are screwed into the base member 4 from above the printed circuit board 7 through the individual support tubes 9. This makes mounting of the printed circuit board 7 complicated and increases the number of parts, resulting in high production cost of the magnetic rotary encoder.
The gap between the magnetic sensor 6 and the circumferential face of the magnetic drum 1 is adjusted as follows. A spacer having thickness equal to the width of the desired gap is placed between the magnetic drum 1 and the magnetic sensor 6. Then the retaining member 5 is pressed against the magnetic drum 1, then screwed to the base member 4. However, the thickness of the spacer is varied by the pressure originating from the pressing force on the retaining member 5, and the gap may be set improperly. It is also to be noted that the problem is not mentioned in Japanese Utility Model Laid-Open No. 62-3012 mentioned hereinabove.